The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In a nuclear reactor such as a pressurized water reactor (PWR) or a boiling water reactor (BWR), a reactor core is contained in a pressure vessel. The core includes a plurality of transversely spaced apart elongate nuclear fuel bundles and control rods. Each of the fuel bundles includes an outer fuel or flow channel typically having a square transverse section. Elongate fuel tubes spaced apart in a conventional matrix are disposed within the flow channel. The bottom of the fuel bundle typically includes a hollow, conical nosepiece or lower tie plate through which water is channeled upwardly through the fuel bundle wherein it is heated by conventional nuclear reactions within the fuel tubes. The top of the fuel bundle is open to allow water and steam to escape. A handle is typically provided for lifting the fuel bundle into or from its position within the reactor core for fuel bundle loading or unloading.
During a typical core refueling operation, 25% or more of spent or burned fuel bundles within the reactor core are replaced with new unused fuel bundles. An upper pool of water is typically located above the reactor core for providing, for example, shielding of radiation from the fuel bundles, and a conventional bridge or gantry is movable over the pool for refueling the reactor core. The bridge includes a trolley mounted grapple which is telescopically extended downwardly through the pool and into the reactor core to grab one of the fuel bundles by its handle at the top thereof, and is then retracted upwardly to remove the fuel bundle. The fuel bundle is continuously maintained under the water to provide shielding thereof as well as for allowing water to flow upwardly through the fuel bundle to cool it. This prevents overheating due to the continuation of nuclear reactions therein which occur at a substantially reduced level than that occurring in an operating reactor core.
In a one bridge refueling system, each fuel bundle, or spent fuel bundle, is removed from the core and is translated one at a time horizontally through the upper pool to an adjacent fuel storage pool and placed vertically in a horizontal array of storage racks to be temporarily stored for up to several years until such spent fuel is then relocated to a long term storage site. A fresh fuel bundle is then transported by the bridge from the fuel storage pool back to the reactor core and positioned therein. Since a typical reactor includes several hundred fuel bundles, a substantial amount of time is required to remove the spent fuel bundles and replace them with fresh fuel bundles.
Furthermore, in a single bridge system, conventional fuel shipping casks can weigh up to about 100 tons. This fuel shipping cask must be individually lifted into the fuel storage pool so that the spent fuel may be inserted therein. The lifting of such a heavy cask involves a risk that the cask may drop and damage the pool and/or the fuel bundles.
Another type of refueling system used in many PWRs and some BWRs uses two bridges with a transfer machine located between the two. One bridge carries fresh and spent fuel bundles individually between the reactor core and the transfer machine, and the other bridge transports spent and fresh fuel bundles between the transfer machine, and the fuel storage pool. The transfer machine then conveys the spent and fresh fuel bundles between the two bridges. In this way, an entire refueling operation may be carried out in less time than using a single bridge since the two bridges and transfer machine may be synchronized with all operating contemporaneously, with each separately carrying a respective fuel bundle therebetween. This system also eliminates the risk of dropping a shipping cask in the storage pool since one of the bridges may be used for transporting fuel between separate pools containing the stored fuel and the shipping casks.
In this two bridge system, the two bridges are typically located in separate buildings, one containing the reactor core, and another containing the fuel storage pool. The common wall between the two buildings must necessarily provide a seal for radiation and pressure between the two buildings and therefore requires a relatively complex transfer tube extending therebetween through which fuel bundles are transferred. The transfer tube is typically oriented either horizontally or inclined so that the passage between the two buildings is relatively small for reducing the complexity of the required seals therebetween. It is desirable to transport the fuel bundles primarily in an upright vertical orientation as they are moved laterally or sideways through the respective pools so that water may continually flow vertically upwardly therethrough for cooling the fuel bundles. The fuel bundles must, therefore, necessarily be temporarily upended or moved from their vertical orientation to the inclined or horizontal orientation for passage through the transfer tube. The conventional transfer machine therefore provides an upending device at each end of the transfer tube to initially rotate a vertical fuel bundle in the required horizontal inclination for passage through the transfer tube, and at the other end thereof another upending device then returns the fuel bundle to the preferred vertical orientation. Suitable seals are provided at both ends of the transfer tube to prevent leakage of the water therethrough.
Since spent fuel bundles have been operating for a considerable amount of time in the reactor core, radioactive corrosion debris is formed within the fuel bundles which is typically broken loose during the transport thereof and falls to the bottom of the respective pools. The radioactive corrosion debris will also fall during the upending operations and during travel through the transfer tube. The radioactive corrosion debris must be cleaned up from time to time which increases the maintenance time and cost in view of the complex transport path required with the two bridge and transfer tube system.
Furthermore, when the fuel bundles are inclined horizontally, natural convection cooling by the water being channeled therethrough is reduced since the vertical path therethrough has been reduced or eliminated. Accordingly, the fuel bundle must be transported relatively quickly through the transfer tube to reduce the likelihood of overheating of the fuel bundles, and additional procedures must be established to provide effective cooling thereof in the event of any failure of the transfer machine while the fuel bundles are upended.